Invited speaker: Per DelsingAffiliation: Chalmers University of Technology, GöteborgTitle: Quantum Optics with Microwaves and Superconducting Circuits

Time and room: 17:15 h, lecture hall IAP

Recently it has become possible to do quantum optics experiments, where propagating microwaves interact with artificial atoms in the form of superconducting circuits [1]. In our case, the artificial atoms are made from transmon qubits, where we utilize also the higher levels of the transmon. In this colloquium I will discuss several such experiments.In the first set of experiments, we embed a transmon artificial atom in an open transmission line. When a weak coherent state is on resonance with the atom, we observe extinction of >99% in the forward propagating field. Addressing the higher levels, it is possible to observe the Autler-Towns splitting, and the Mollow triplet. Using the Autler-Towns splitting we demonstrate how photons can be routed efficiently and fast on-chip [2]. By applying a control tone, we also observe a giant cross-Kerr effect [4]. Furthermore we study the statistics of the reflected and transmitted radiation and we demonstrate antibunching in the reflected field and superbunching of the transmitted field.
In a second set of experiments, we embed a transmon at a distance from the end of an open transmission line, which acts as a mirror[5]. By tuning the wavelength of the atom, we effectively change the normalized distance between atom and mirror, allowing us to effectively move the atom from a node to an antinode of the vacuum fluctuations. We probe the strength of vacuum fluctuations by measuring spontaneous emission rate of the atom.
[1] I.-C. Hoi et al. New Journal of Physics 15, 025011 (2013)
[2] I.-C. Hoi et al. Physical Review Letters 107, 073601 (2011)
[3] I.-C. Hoi et al. Physical Review Letters 108, 263601 (2012)
[4] I.-C. Hoi et al. Physical Review Letters 111, 053601 (2013)
[5] I.-C. Hoi et al. Nature Physics, 11, 1045 (2015)

The evolution of an isolated quantum system is unitary. This is simple to probe for small systems consisting of few non-interacting particles. But what happens if the system becomes large and its constituents interact? In general one will not be able to follow the evolution of the complex many body eigenstates. Ultra cold quantum gases are an ideal system to probe these aspects of many body quantum physics and the related quantum fields. Our pet systems are one-dimensional Bose-gases. Interfering two systems allows studying coherence between the two quantum fields and the full distribution functions and correlation functions give detailed insight into the many body states and their non-equilibrium evolution. In our experiments we study how the coherence created between the two isolated one-dimensional quantum gases by coherent splitting slowly degrades by coupling to the many internal degrees of freedom available [1]. We find that a one-dimensional quantum system relaxes to a pre-thermalisatized quasi steady state [2] which emerges through a light cone like spreading of ’de-coherence’ [3]. The pre-thermalized state is described by a generalized Gibbs ensemble [5]. Finally we investigate two distinct ways for subsequent evolution away from the pre-thermalized state. One proceeds by further de-phasing, the other by higher order phonon scattering processes. In both cases the final state is indistinguishable from a thermally relaxed state. We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by long lived excitations (quasi particles).

Gauge theories are fundamental to our understanding of interactions between the elementary constituents of matter as mediated by gauge bosons. However, computing the real-time dynamics in gauge theories is a notorious challenge for classical computational methods. In the spirit of Feynman's vision of a quantum simulator, this has recently stimulated theoretical effort to devise schemes for simulating such theories on engineered quantum-mechanical devices, with the difficulty that gauge invariance and the associated local conservation laws (Gauss laws) need to be implemented. Here we report the first experimental demonstration of a digital quantum simulation of a lattice gauge theory, by realising 1+1-dimensional quantum electrodynamics (Schwinger model) on a few-qubit trapped-ion quantum computer. We are interested in the real-time evolution of the Schwinger mechanism, describing the instability of the bare vacuum due to quantum fluctuations, which manifests itself in the spontaneous creation of electron-positron pairs. Our work represents a first step towards quantum simulating high-energy theories with atomic physics experiments, the long-term vision being the extension to real-time quantum simulations of non-Abelian lattice gauge theories.

It is well known fact that various phase transitions in condensed matter can by triggered by external parameters such as temperature, pressure, electric field or magnetic field. Finding systems that show phase transitions triggered by external stimulation of light became a particular interesting field of research.
Advanced nonlinear optical methods such as ultra-broad band pump-probe spectroscopy open new ways of controlling ultrafast dynamics in complex solid-state materials on unprecedented timescales. In quantum materials, finding new ways of manipulating the complex interplay of electronic phases or effectively tuning electronic interactions opens new avenues in controlling physical properties and designing new functionalities.
I will show how we investigate different scenarios like the balancing between competing phases triggered by ultrashort light pulses or possibilities of dynamical stabilization of new states of matter in periodically driven light fields. In particular I will discuss the remarkable possibilities to induce superconductivity in high temperature cuprate superconductors by melting competing “stripe”-order [1] or even promoting it to temperatures far above Tc; for some underdoped materials even up to room temperature for a few picoseconds [2,3]. Possible light-induced superconductivity in the doped fullerides K3C60 [4] will serve as important example that inducing such intriguing effects is a more general effect and not restricted to the rather specialized class of cuprate systems.
[1] D. Fausti et al. Science 331, 189 (2011).
[2] S. Kaiser et al. Phys. Rev. B 89, 184515 (2014).
[3] W. Hu et al. Nature Materials 13, 705 (2014).
[4] M. Mitrano et al. Nature 530, 461 (2016).

The Hofstadter butterfly [1] is the intricate self-similar structure of subgaps that the single energy band of a charged particle hopping on a 2-dimensional lattice develops as a magnetic field perpendicular to the lattice is turned on. It has inspired physicists for 40 years (e.g., an important role in the exploration of the quantum Hall effect), and now enjoys a renewed interest as experiments might finally be close to observing it. In case the charged particle has several internal states, the spectrum as a function of magnetic field is a multiband Hofstadter butterfly, where each energy band develops a set of subgaps. We show that besides developing subgaps, the bands of topologically nontrivial (e.g., Chern) insulators must also flow in energy as the magnetic field is tuned, because eigenstates flow across topological gaps at a steady rate. We thus connect the global topology of multiband Hofstadter butterflies, i.e., the pattern in which bands flow into each other, with the topological invariants of the underlying lattice Hamiltonians. Our results also apply to quantum walks, and other periodically driven systems, where we obtain a simple formula for the Rudner topological invariant [2], which has potential to be directly measured.